1. Field of the Invention
The present invention relates to a method, an apparatus, and a program for correcting pixel signal values of digital images obtained by digital still cameras.
2. Description of the Related Art
Conventional image processing apparatuses that generate ID photos to be utilized in ID's, driver's licenses, passports and the like input images of the upper bodies of subjects (people) by photography, and output prints. These image processing apparatuses perform corrections on images, so that even if input conditions vary, prints are generated with consistent qualities. Specifically, a method has been proposed, in which skin colored regions of an image are specified by a mouse or the like, and the skin colors are corrected (as disclosed in, for example, U.S. Pat. No. 5,296,945).
In addition, in image processing apparatuses that generate ID photos, the areas within images that faces are positioned at are within a certain range, because photography is performed while adjusting the heights of chairs that the subjects sit in. Therefore, methods have been proposed in Japanese Unexamined Patent Publication Nos. 10(1998)-221773 and 2000-261650, in which facial areas to be corrected can be automatically extracted, based on data regarding these positions and skin colored pixel areas. In these methods, characteristic values are calculated employing average color densities of the facial areas. Then, correction coefficients are determined, based on an amount of difference between the average color densities of the extracted facial area and an average color density of a facial area within a preset reference image. Finally, image data is corrected employing the correction coefficients.
Each of the above methods is able to obtain favorable correction results in both a brighter direction and a darker direction in cases in which the dynamic ranges of recording media, with which the images are to be reproduced, are wide. However, in image data sets, such as those obtained with digital still cameras and only have an 8 bit memory range, the R, G, and B values can only be represented within a range of 0 to 255. Any data beyond this range is lost. If regions having RGB values that exceed (255, 255, 255) exist within a facial region of an overexposed image, attempts to correct the densities will only yield gray pixels, therefore a favorable image is not always obtainable by correction. Even if the image is not overexposed, there are many cases in which corrections toward a darker direction, administered to images that include faces, result in faded images.